A semiconductor light-emitting element such as a light-emitting diode (LED) has advantageous features of (i) being small in size, (ii) consuming less electric power, and (iii) stably emitting light with high luminance. In recent years, there has been tendency to replace a lighting apparatus such as an incandescent lamp with a lighting apparatus including a light-emitting device made up of an LED that emits white light.
An example of the LED that emits white light is one in which a blue LED is combined with a Ce-activated YAG fluorescent material expressed by a composition formula (Y,Gd)3(Al,Ga)5O12:Ce.
In the light-emitting device thus configured, white light is obtained by mixing blue light emitted by the blue LED and yellow light emitted by the Ce-activated YAG fluorescent material. In this configuration, the Ce-activated YAG fluorescent material is excited highly efficiently in a wavelength range between 440 nm and 470 nm, in which the blue LED emits light highly efficiently. This allows the light-emitting device itself to emit light with high efficiency.
However, the Ce-activated YAG fluorescent material has narrow full width at half maximum of emission spectrum, and this leads to lack of a red component of the light-emitting device. Therefore, in a case where the light-emitting device is employed as a household lighting apparatus etc., there occurs a problem such as that the human skin color looks unnatural.
Specifically, the light-emitting device has an average color rendering index (Ra) of approximately 70, in a color temperature range defined by neutral white and lamp color used in lighting apparatuses. In particular, the light-emitting device has a special color rendering index (R9) of approximately −40, which indicates how red looks. These indexes show that, in a case where the light-emitting device is employed as a lighting apparatus, red will look extremely improperly. Note that “Ra” is an index indicating how faithfully test light reproduces a test color, where a color seen under reference light is indicated by 100. In particular, “R9” is the special color rendering index for red.
In recent years, a configuration for improving Ra and R9 of white light from a semiconductor light-emitting device has been proposed (for example, see Patent Literature 1). In the configuration, a red fluorescent material such as a nitride fluorescent material is further incorporated with the blue LED and the yellow fluorescent material such as the Ce-activated YAG fluorescent material so that the narrow full width at half maximum of emission spectrum of the Ce-activated YAG fluorescent material is compensated by red light emitted by the red fluorescent material. This enhances uniformity of the emission spectrum of the fluorescent materials, and accordingly Ra and R9 of white light emitted by the semiconductor light-emitting device are improved.
Further, in recent years, a fluorescent material has been proposed which is made up of, as host crystal, CaAlSiN3 crystal containing at least Li, Ca, Si, Al, O, N, and Ce (for example, see Patent Literature 2). It is disclosed that the fluorescent material is suitably used to provide a white LED by being combined with a blue LED.